Emergency lighting has been employed for several decades, for example to provide power to one or more light sources for illumination of the path of egress from a building or facility. Emergency lighting is required in industrial, commercial, and institutional buildings as part of the safety equipment. Emergency lighting relies on a limited backup power source for example a battery, to supply power to the light source(s). Emergency lighting unit (sometimes referred to as an “emergency ballast”) is designed to energize the light source(s) exclusively during periods of AC power failure, when the ballast is said to be in “emergency mode” (EM), and may be combined with a conventional lighting unit (sometimes referred to as an “AC ballast”). The emergency lighting unit may sense the absence of the AC power and use the backup power source and dedicated electronic circuitry to energize the light source(s) during a limited period of AC power failure. In the USA, the required emergency lighting period is at least 90 minutes, while in Europe, e.g., it is 180 minutes, during which the emergency illumination level should not decline to under 60% of the initial level, as set for battery-powered emergency lighting systems by the life safety codes (e.g., section 7.2 of NFPA-101 and NEC 700.12).
Recently, light-emitting diodes (LEDs) have become more prominent in the market as a main light source for an occupied space. LEDs offer a viable alternative to traditional fluorescent, HID, and incandescent lamps. Functional advantages and benefits of LEDs include high energy conversion and optical efficiency, durability, lower operating costs, and many others. Recent advances in LED technology have provided efficient and robust full-spectrum lighting sources that enable a variety of lighting effects in many applications. These advantages are leading to the introduction of LEDs into a wide variety of applications and context. In particular, LED light sources are now being developed for use in emergency lighting systems.
Since emergency lighting depends on a limited power source (e.g., one or more batteries), it becomes critical to have the ability to control the power supplied to the light source(s). Typically, power conversion circuits for emergency lighting implement constant voltage control or constant current control. In the approach of constant voltage control, the output power is decreased as the output impedance increases. In the approach of constant current control, the output power is increased as the output impedance increases. If the output power is controlled by simply adopting the approach of constant voltage control or the constant current control, the cost is relatively low and the circuit design is relatively uncomplicated, but the power is easily varied and optimal use is not made of the energy available by the backup power source.
For example, emergency lighting systems based on LED loads typically attempt to provide a constant current to the LED load, which, by virtue of the heat dissipated by the LED load elements, lead to a decreasing load voltage profile, “naturally” translated into a monotonically decreasing output power profile. However, this inherently leads not only to a lack of control of the gradual decrease in the output power, but also fails to conserve input energy (e.g., from one or more batteries) via a fully controlled output power profile, which is vital for an emergency lighting system that needs to comply with strict code regulations.
Thus, there is a need in the art to provide an emergency lighting driver which can provide a controlled power output profile which can meet regulatory requirements for emergency lighting levels over time, while also maximizing the use of the available energy from the backup power source, thus providing the possibility of using fewer and/or smaller batteries.